Genome Property Definition Page

Accession

GenProp0016

Name

selenocysteine system

Type

SYSTEM

Description

The property of incorporating selenocysteine as an alternate amino acid during protein translation from mRNA. The site of incorporation often aligns to an active site Cys residue of a homologous protein in a selenocysteine-negative species. One required element is a selenocysteine-specific tRNA (selC) to recognize a TGA codon in the appropriate context. Two required proteins are a selenocysteine-specific elongation factor (SelB), and a selenium donor protein (selD) that may participate in other systems in addition or instead (selenouridine biosynthesis, and selenium-dependent molybdenum hydroxylase maturation). Finally, a mechanism for converting the serine placed on selenocysteine-specific tRNA into selenocysteine is required. This is achieved by seryl-t RNA(sec) selenium transferase (SelA) in bacteria, but in a two-step process starting with phosphorylation in the Archaea and Eukaryota.
Note that SelD may itself be a selenoprotein, and so may appear truncated by a stop codon, thus not triggering the rule to assign the selenocysteine property automatically. Before the state of this property is set to "YES", an example of a potential
selenocysteine-containing protein must be manually annotated. Commonly observed selenoproteins include the selenium donor protein SelD (selenide, water dikinase) and subunits of formate dehydrogenase, thioredoxin reductase, glycine reductase, heterodisulfide reductase, methylviologen-reducing hydrogenase and glutathione peroxidase.
In certain species all components of the system can be found save the SelC tRNA (including examples) even after a manual search. These are given the state "PARTIAL" to indicate the possibility that either the ability to incorporate selenocysteine has been lost (and the examples are actually truncated or not expressed) or an as-of-yet undescribed tRNA system exists in these species.
The selenium donor protein seems to occur in some species that lack the selenium-specific tRNA, elongation factor, and transferase. This usually reflects the incorporation of selenium into certain tRNAs. Note also that selenium is sometimes used to label proteins in vitro, so not every literature reference to selenocysteine implies incorporation during translation.
An analogous system, discovered more recently, incorporates pyrrolysine at specific TAG codons.